It is particularly important to be able to determine the stopped position of a rotor, in particular so as to be able to apply the most appropriate control signals immediately on starting the synchronous rotary electrical machine.
In most prior art systems, the synchronous rotary electrical machine is therefore equipped with sensors responsive to the angular position of the rotor. Recent research has developed methods for measuring the stopped angular position of the rotor without using position sensors. For example, European Patent Application EP 1 160 966 discloses a method of this type applied to a rotary electrical machine including a permanent magnet rotor and a stator which has an excitation winding for each phase. The windings of this rotary electrical machine are connected in a star configuration.
The method disclosed in EP 1 160 966 comprises the following steps:                applying a pair of successive voltage pulses of equal and opposite predefined values to the terminals of a first winding and then a second winding, respectively,        for each pulse and after a predefined time interval, measuring a voltage at the terminals of a measuring resistor connected between the neutral of the star configuration and ground,        for each pair of pulses, determining the difference between the two voltages measured at the terminals of the resistor for the first winding and then for the second winding, respectively,        establishing possible angular positions of the rotor from prestored data representing the correlation between those angular positions and the differences previously determined, and        selecting from the possible angular positions previously established the one that is the most probable.        
Although satisfactory, that method has a number of drawbacks. In particular, the architecture of the rotary electrical machine and its control unit must be modified. In addition to an inverter adapted to apply three-phase voltages to the terminals of the windings of the rotary electrical machine, the control unit must be equipped with an independent generator of pairs of pulses to be applied to the terminals of the windings. The motor must be modified to connect a voltage sensor between the star neutral and ground.